The present invention relates generally to a method of determining the pressure, temperature and optionally flow in the medical area, especially to in situ measurement of the intracoronary pressure, distally of a stricture using a guide wire having a pressure sensor at its distal end.
In order to determine and investigate the ability of a specific coronary vessel to supply blood to a heart muscle, i.e. the myocardium, there is a method through which the intracoronary pressure distally and proximal of a stricture are measured. In the method the so called Fractional Flow Reserve (see xe2x80x9cFractional Flow Reservexe2x80x9d Circulation, vol. 92, No 11, Dec. 1, 1995, by Nico H. J. Pijls et al.) is used. Briefly FFRmyo is defined as the ratio between the pressure distally of a stricture and the pressure proximal of a stricture, i.e. FFRmyo=Pdist/Pprox. The measurement of the distal pressure is made in the vessel using a micro-pressure transducer, and the proximal pressure is the arterial pressure.
In WO 97/27802 there is disclosed a sensor for temperature and pressure measurements having a pressure sensitive resistor (referred to as an active resistor) and a temperature sensitive resistor (referred to as a passive resistor). These resistors are part of a Wheatstone bridge comprising a total of four resistors and three xe2x80x9cparasiticxe2x80x9d cable resistances. Two different measurements are performed in this bridge, namely a measurement of a differential output voltage (potential difference between the active and passive circuit) which is used for pressure measurement and single ended output voltage (voltage across the passive circuit), which is used for temperature measurement. Only the passive and the active resistor are located on the actual sensor chip that is inserted into the body. The remaining resistors in the bridge are located externally, suitably in the control unit, and are coupled to the chip via electrical leads running inside a guide wire on the distal end of which the chip is located. This system functions appropriately when the environment is such that the properties of the electrical leads are not affected. However, in view of the fact that the leads have extremely small dimensions and are relatively long, the resistance of the leads could vary considerably if the fraction of the length leads that are exposed to the body temperature varies, namely, if the guide wire needs to be manipulated over distances of more than a few centimeters. This effect is obtained in general when the temperature surrounding the guide wire changes e.g. when flushing with saline having a temperature differing the from body temperature. These effects will have an impact on the temperature and pressure signals, and should be compensated for. In particular the read-out value of the temperature sensor (the passive resistor), which is also used for temperature compensation, will be incorrect. Thus, it is desirable to obtain a correct temperature read-out. One way of achieving this would be to provide separate leads coupled so as to enable a direct reading of the voltage drop across the passive (temperature sensitive) resistor. However, this would require the provision of five electrical leads inside the guide wire, and there is simply not space enough to make this possible.
In U.S. Pat. No. 5,715,827 (Cardiometrics), which is incorporated in its entirety herein, there is disclosed apparatus for pressure measurements using two resistors provided on a flexible diaphragm. The resistors are mounted so as to have opposite response to pressure, i.e. one resistor gives a positive signal and the other a negative. In the patent it is stated that this arrangement provides a temperature compensation for the signal. However, this arrangement has the same inherent problem with cable resistances affecting the signal.
The object of the invention is therefore to provide a measurement method using a single unit having as few electrical leads as possible, namely only three, and allowing compensation for variations in resistance of the leads coupling the resistors in the sensor to the control unit, in order to obtain a reliable measurement.
The object outlined above is achieved according to the invention with the method of measurement described herein. The method comprises the excitation of the sensors so as to produce two distinguishable signals. At least a component of at least one of the signals is representative of the cable resistance and can be used for compensation purposes.
In a second aspect of the invention there is provided an apparatus suitable for such measurements having temperature compensation means. The compensation means comprises circuitry for selectively and independently registering a resistance value for the passive resistor on the chip.
Further scope of application will become apparent from the detailed description given hereinafter.
Nevertheless, it should be understood that the detailed description and specific examples, while describing a measurement method, are given as an illustration only. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.